The present invention relates generally to digital motor speed regulator systems providing method and apparatus for controlling the flow of power from an AC source to a load such as a DC motor. More specifically, the present invention relates to a digital control system for the direct digital firing of controllable rectifiers placed between an AC source and a DC motor for controlling the conduction of such rectifiers.
Motor control systems of the type described herein, frequently use power amplifiers having control rectifiers which vary the control of electrical energy between an alternating current source and a drive motor. Control rectifiers are well known in the art and generally consist of a family of devices which present a relatively high impedance to the flow of electrical energy until they are forward biased by a firing signal applied to a gate electrode. At the time of conduction, a control rectifier normally provides a very low impedance to the flow of current and continues to conduct current until it is back biased and/or the level of the current flowing to the rectifier is decreased below a minimum holding level necessary to keep the rectifier conducting. The family of control rectifiers being discussed generally includes semiconductor devices such as silicon controlled rectifiers (thyristors) and other devices such as ignitrons and thyratrons.
In systems, such as the type being described, the amount of power transferred to a system load or DC motor is controlled by varying the duration of the conduction of controllable rectifiers. Generally speaking, the duration of conduction of the controllable rectifiers is a function of the point during the AC wave form in which they are initiated into conduction. This point is referred to as the firing angle.
In the past, systems for controlling the conduction of controllable rectifiers, have generally been accomplished by using analog control devices to perform the regulating functions required and converting the analog signal into digital values to fire the rectifiers. In these types of systems, the firing circuits act in response to an input signal indicative of desired power to generate a firing pulse at the appropriate firing angle. Generally speaking, the firing angle is directly proportional to the input signal. Analog systems of the prior art generally operate in response to an input signal whose magnitude indicates the desired firing angle.
In recent years, however, with the development of digital techniques and hardware, engineers have become interested in utilizing digital circuitry in these types of control systems. The application of digital techniques to such control systems is particularly advantageous where the system requires that degree of accuracy, reliability or drift free operation which is available only with digital circuitry. Hence, it is becoming fairly common to replace elements of an analog system with functionally equivalent digital circuitry. One such known digital control system for controlling the conduction of controllable rectifiers, is disclosed in U.S. Pat. No. 3,601,674 entitled "CONTROL SYSTEM FOR FIRING SCR's IN POWER CONVERSION APPARATUS" to Albert F. Koch and assigned to the assignee of the present invention. In that patent a digital control system is disclosed for controlling the flow of power through controllable rectifiers from a multiphase AC source to a load. That system includes a firing circuit for each phase, wherein, each firing circuit comprises a reversible counter and a digital comparator. Phase detection logic is incorporated which examines the three phases of the AC source so as to synchronously initiate a control interval for an appropriate rectifier by presetting a predetermined positive or negative digital number into the reversible counter associated with each phase. The reversible counter then counts down, if the preset number is positive, or up if the preset number is negative, during the control interval. During counting, a digital speed error signal derived from a previous comparison of a digital command with a digital feedback signal indicative of motor speed is continuously compared with the contents of the reversible counter in a digital comparator. When the error exceeds the contents of the reversible counter, a firing pulse is generated which is applied to a positive or negative poled rectifier firing the respectively poled rectifier in accordance with the positive or negative number.
Another prior art system of the digital type has been described in two papers: the first by R. D. Jackson and R. D. Weatherby, entitled "DIRECTION DIGITAL CONTROL OF THYRISTOR CONVERTERS" in IFAC Symposium on Control and Power Electronics and Electrical Drives, October 1974, held in Dusseldorf, Germany preprint Volume I, pages 431-441; and the second paper by F. Fallside and R. D. Jackson entitled "DIRECT DIGITAL CONTROL OF THYRISTOR AMPLIFIERS" in the Proceedings of The Institute of Electrical Engineers - Control and Science, Volume 116, No. 5, pp. 873-878 May, 1969.
In the above mentioned papers, the authors describe a study of a laboratory system of the direct digital control type to demonstrate the feasibility of direct digital control of controllable rectifiers such as silicon control rectifiers. In this system a programmed digital computer is employed to control the firing of rectifiers through an interface apparatus to control a resistive-capacitive load by the generation of firing pulses generated from the computer. The digital computer computes the firing angle which specifies a time to fire a particular rectifier. The system is synchronized to the phase to neutral crossings of the phase voltages of the AC source, such phase crossings initiating command signals for a sample and hold circuit for an analog to digital converter which measures the system load output voltage.
At the end of the analog to digital conversion, a pulse is generated from the converter as an interrupt signal to the computer. This pulse starts the firing angle calculation. Subsequent to the initiation of the interrupt signal, the computer reads the analog to digital converter at a specified time after the phase crossover instant. The computer then proceeds to calculate the firing angle or firing instant for the rectifier utilizing what is referred to as a given firing law. In this calculation, a control signal or value is developed which is continuously compared with a linear lookup table, containing values defining the firing law, until a match exists between the control signal and the contents of the table. When a comparison is reached, a firing pulse is generated by setting up the address of the rectifier to be fired and strobing a firing signal to the rectifier.
While the authors did demonstrate the feasibility of direct digital control of controllable rectifiers, they also recognized that various practical difficulties were experienced in setting up a system of this type. This was apparent in the most obvious implementation in these experiments of the use of lookup tables which takes a considerable amount of computer time and severely restricts the amount of additional computation time needed by the computer in operating a real time system of this type.
Further, it is to be recognized that direct digital drive control systems must sample the load output voltage at a specified instant in time as related to a detected phase interval of the AC source, then perform the necessary firing angle calculation and fire a selected rectifier within that detected interval, and at a time early enough in the specified interval in order to accurately and fully control the retardation of the firing angle to obtain maximum transfer of power to the load.
The above described systems did not function to perform as total motor speed control systems for controlling a variable speed reversible drive motor. In analog systems the manner of controlling the speed of a DC motor, when operating in either continuous or discontinuous current mode, is well known as is the manner of reversing the direction of the motor. It is also known that one criteria for reversing the direction of a DC motor is that the motor current be zero at the time of reversal. In analog systems, in order to make this reversal it is first necessary to detect when the current is zero and then to wait a specified safe period before reversing the motor. Additionally, it is also known in analog motor drives systems that two feed-back loops are required, one to control the motor when it is in continuous current mode operation and one to control the motor when it is in discontinuous current mode operation. These two loops provide separate gains to the system dependent upon the mode of operation. This method of operation in analog control systems has proven to be somewhat unsatisfactory in DC motor drives systems, particularly when it is desired to achieve a high degree of constant motor speed operation under very light load conditions. Thus, it is desirable to provide a DC motor speed drive and regulator system which enhances the overall operation of the system by providing a regulator which measures system parameters and accurately calculates motor speed, determines the mode of operation of the system, and promptly sets system gains, determines the motor direction of rotation and changes the direction of motor rotation instantaneously without a delay if required to do so.
It is, therefore, an object to provide an improved control system for the direct digital firing of rectifiers to control a DC motor.
It is further object to provide a digital regulator and motor control system for monitoring system parameters and calculating a firing angle from such parameters to generate a firing pulse to control a rectifier supplying power to a motor to provide maximum delivery of power to the motor.
Still a further object is to provide a method in a digital regulator motor control system for reading system parameters and determining the mode of operation of the system to control the overall system gain in accordance with the determined mode and to calculate a firing angle from the parameters and the determined gain to fire a rectifier at a specified instant in accordance with the calculated firing angle.
It is still another object to provide a direct digital firing control system, for controlling a DC motor, having a data processor which performs calculations in response to a firing pulse generated by the system to enable the processor to determine the direction of rotation of the motor and a desired direction of rotation of the motor and to calculate a firing angle specifying the instant for the firing of a selected rectifier in accordance with the determined direction of motor rotation, such firing angle calculation being performed in sufficient time to allow the rectifier to be fired early enough in a phase interval of the AC source to deliver maximum power to the DC motor.
The foregoing and other objects are achieved in accordance with the present invention through the provision of a control system for direct digital firing of controllable rectifiers through the implementation of a programmed data processor and interface means in which the processor monitors system parameters proportional to desired motor speed, actual motor speed, and motor direction, and motor current and calculates a firing angle specifying an instant at which a selected rectifier is to be fired to deliver maximum power to a DC motor in accordance with the direction of motor rotation. The interface means includes logic or counter means for receiving the calculated value of firing angle from the processor to generate a firing pulse for a selected rectifier when the counter achieves a specified count while simultaneously applying the firing pulse to the processor as an interrupt to signal the processor to calculate a new firing angle for a subsequent rectifier to be fired.
In the present invention the complete calculation of firing angle, the selection of a rectifier to be fired, and the determination and selection of the mode of operation of the system, and the direction of motor rotation are all performed in sufficient time to allow the processor to generate a firing angle for use in firing the selected rectifier in a predetermined phase interval of the AC source to allow maximum delivery of power to the DC motor.
The present invention extends the state of the art of DC motor control systems utilizing a data processor to provide digital regulation of rate of change of motor current, speed and direction, including compensation for motor and system time constants and compensation for continuous and discontinuous current modes of operation and further provides means of adjusting transient performance as determined by the processor.
Clock means in the interface means monitors the AC power source and provides indicia to the processor representative of a time interval of the AC source referenced to the phase crossings of each of the phases and further specifies value proportional to time within each of the intervals. The indicia are utilized by the computer to calculate a time for the processor to load the calculated value of firing angle into the counter of the interface means and to also calculate the selection of the proper rectifier to be fired.